While it might be easy to forget about the small accessory ligament that anchors the superficial digital flexor tendon to the upper foreleg bone (radius), acute desmitis (ligament inflammation) of this structure has now been shown to be a significant cause of pronounced, transient lameness in Thoroughbred racehorses. Johanna Reimer, VMD, Dipl. ACVIM, of Rood & Riddle Equine Hospital in Lexington, Ky., presented her findings on this injury at the 2003 American Association of Equine Practitioners convention in New Orleans, La., on Nov. 20.

Previous research had described injury to the accessory ligament of the superficial digital flexor tendon (AL-SDFT) in racehorses, sport horses, and pleasure horses, with the problem often being accompanied by other problems such as synovitis, tendonitis, and proximal suspensory desmitis. The prognosis was usually fair in previous studies, Reimer noted, with treatments including intrathecal (within the sheath) hyaluronic acid and corticosteroids along with controlled exercise at six months post-injury.

She diagnosed the condition in 27 Thoroughbred racehorses and one American Saddlebred (which was left out of the analysis) after they were presented with the following clinical signs:

  • Transient lameness in eight horses; two had been pulled up during races;
  • Persistent effusion (constant swelling) of the carpal sheath in 12 horses;
  • Palpable thickening behind the middle of the knee (all cases);
  • Recurrent effusion of the carpal sheath in four horses; and/or
  • Apparent thickening of the proximal (upper) SDFT that looks like a bowed tendon in 13 horses.

One horse was a 2-year-old, seven were 3-year-olds, 11 were four to five years old, and eight were six years old or older. Nineteen horses had their right forelimbs affected, seven horses had their left forelimb affected, and one horse had the injury in the left fore, then in the right upon his return to racing.

Ultrasound examination starting at the lower edge of the chestnut revealed a "diffuse decrease in echogenicity (indicating fluid buildup throughout the ligament) and hypoechoic appearance to the ligament, or a discreet area of fiber tearing," said Reimer. "The opposite AL-SDFT was always imaged to better illustrate the anatomy and abnormalities. This makes a great split screen for showing the injury to owners."

Reimer found a few other problems in the area during ultrasound examination, including:

  • Thickening (swelling) over the SDFT behind the upper part of the knee in 13 horses;
  • Variable degrees of effusion within the lower carpal canal (the depression running down the back of the knee in which the flexor tendons lie). No effusion--three horses; mild effusion--eight horses; moderation effusion--13 horses; marked effusion--four horses;
  • Mild decrease in echogenicity (some loss of density) in the origin of the suspensory ligament in four horses;
  • Mild decrease in echogenicity of the SDFT in one horse;
  • Chronic bowed SDFT in one horse; and
  • Mild synovial thickening of the carpal sheath in one horse.

Treatment Success

Recommendations were that all horses go on a regimen of four to eight weeks of stall rest with hand or under-tack walking, then four to eight weeks of turnout, followed by a gradual return to training, Reimer reported. She knew of no hyaluronic acid or corticosteroid injections for treatment in these cases.

She was only able to examine 11 of the 27 horses after the initial diagnosis to assess recovery. Of those 11, eight had more normally sized AL-SDFT and improved appearance of the injured AL-SDFT on ultrasound (normal or near-normal size and echogenicity), although with non-parallel fibers (fibers are parallel in a normal ligament).

Three of the horses had swimming or water treadmill exercise at various times during recovery (soon after injury or after 2 1/2 months), and all three had poor healing or re-injury of the AL-SDFT. Two of these horses healed properly following a period of stall rest and hand walking; the third was not re-scanned by Reimer.

Reimer said that four horses had been retired. Twenty-two returned to training, with one dying of unrelated causes before racing. Nineteen (86%) returned to racing after an average of 8.5 months (range--two weeks to 16 months). One horse injured the other AL-SDFT during his first start back, but raced again six months later. Three horses started three times after recovering, three had four starts, 11 horses had at least five starts, and three only recently returned to racing.

The median of the average earnings per start before injury for this group was $3,686; after recovery, it was $719. However, Reimer noted that purses are lower for older horses, and post-injury performance might not be the only factor decreasing the earnings.

Summing Up

"Acute desmitis of the AL-SDFT might be an under-diagnosed condition in the Thoroughbred racehorse," Reimer said. She also noted that the condition was more prevalent in horses at least four years old, tended to appear in the right forelimb (at least in these American racehorses), and was not associated with other problems in the carpal canal region.

"A history of transient lameness and/or carpal sheath effusion should alert the clinician to the possibility of AL-SDFT injury; however, it was found in this study that the presence of peritendinous (near the tendon) edema in the proximal metacarpal region (upper cannon bone), mimicking the appearance of a bowed tendon, might be the only complaint and should prompt examination of the AL-SDFT," Reimer stated. "Palpable thickening of the tissues in the palmar medial carpal region (behind the knee) seems to be a fairly consistent finding."

She went on to suggest a treatment of four weeks of stall rest and hand walking followed by four weeks of small paddock turnout if follow-up ultrasound examination shows significant improvement after the four weeks of rest. Or, the horse can have eight weeks of stall rest with hand walking. Then there should be a gradual return to training. She suggested that swimming and water treadmill exercise be avoided during recovery, and she recommended a repeat ultrasound examination at two months post-injury before returning the horse to training.

"If the ligament seems of uniform and normal echogenicity, jogging may resume with an anticipated return to racing as early as six months post-injury in some cases," she said.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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